The hydraulic mean depth of a circular sewer of internal diameter 100 cm, running in fully occupied cross section is given by:

Correct option is B

Hydraulic mean depth = Area of flow/Perimeter of flow

​$$\begin{aligned}&= \frac{\pi}{4} \frac{D^2}{\pi D} \\&= \frac{D}{4} \\&= \frac{D \times 100}{4} = 25 \, \text{cm}\end{aligned}$$​